HDDs (Hard Disk Drives) and flash memories are used as storage media for a storage subsystem. When destaging processing is executed for writing data in a cache memory to storage media in order to process the relevant data in accordance with an I/O command from a host computer in the above-described type of storage subsystem, pieces of update data may exist at separate locations in cache slots in the cache memory.
If an HDD is used as a storage medium in the above-described situation, the pieces of the update data existing in the cache slots and the sparced data existing between the pieces of the update data are put together to form a continuous data group, which is then written to the HDD (see Non-patent Document 1). In this case, it is only necessary to issue an I/O command once without issuing the I/O command for each piece of the update data and, therefore, it is possible to reduce processing time required for seek and rotational delay.
On the other hand, if a flash memory is used as a storage medium, a method of extracting only the pieces of the update data existing in cache slots in the cache memory, writing the extracted pieces of the update data separately to the flash memory, and thereby reducing the data amount used per writing is adopted.